1. Field of the Invention
This invention relates to an aqueous ink for inkjet recording.
2. Description of the Related Art
Ink comprising various kinds of water-soluble dye dissolved in an aqueous medium has been generally employed as an inkjet recording ink. In recent years, ink comprising a pigment which is dispersed in an aqueous medium has been proposed. The reason is that, as compared with the ink employing water-soluble dye, the ink employing a pigment is excellent in water resistance and in resistance to light.
Many kinds of the conventional ink using pigments are formulated so as to suppress the wetting properties of ink to the surface of paper by mainly limiting the permeability of ink, thereby making it possible to retain the printed ink in the vicinity of the surface of paper and to secure the quality of printing. These kinds of ink however are accompanied with a problem that they are not always applicable to every kinds of paper. More specifically, the bleeding of ink occurs prominently depending on the kinds of paper. Especially, in the case of recycled paper to be manufactured from the mixture of many and unspecified kinds of paper, the difference in wetting properties of ink to the kinds of paper would give a great influence to the generation of bleeding, thereby destabilizing the quality of printing. Further, in the case of conventional pigment ink, it takes a long time for drying printed portions due to the specific nature of ink composition, resulting in the mixing of colors between the neighboring colors especially in multi-color printing such as color printing. Further, there is a problem that since pigment leave on the surface of paper after printing, anti-fretting properties of printed matter are caused to deteriorate.
With a view to overcome these problems, it is proposed in JP-A 9-111165 (KOKAI) and U.S. Pat. No. 5,156,675 to incorporate a glycol ether-based penetrating agent to enhance the permeability of ink to paper. In the case of the conventional pigment ink, it has been generally practiced to disperse pigment in an aqueous medium with a dispersant such as surfactants, polymeric dispersant, etc. The glycol ether-based penetrating agent however is accompanied with problems that it may deteriorate the effects of dispersant and hence, if the glycol ether-based penetrating agent is incorporated in ink at a large content, the dispersion stability of pigment decreases with time.
No one has succeeded as yet to find out an aqueous ink for inkjet recording, which is suited for printing on the ordinary paper, excellent in quick-drying property and in quality of printed images of printed matter, free from clogging of ink during the intermittent discharge of ink, and excellent in dispersion stability with time of pigment.
Incidentally, in the case of volatile oily ink, there has been proposed to employ 3-methoxy-3-methyl-1-butanol as a solvent as disclosed in JP-A 2005-104139 (KOKAI). Since this oil ink is used in an oil ball-point pen, the vapor pressure thereof at 25° C. is regulated to 0.001 mmHg or more.